Generally, in a hot rolling process, continuous rolling refers to the process in which a trailing portion of a steel sheet that is undergoing rolling and a leading portion of a steel sheet to subsequently undergo rolling are joined to thereby continuously perform rolling. This is in contrast to the existing method by which steel sheets respectively rolled into coils are separately rolled.
Since steel sheets may be continuously rolled using the continuous rolling method (even if the coil is changed), productivity is improved. Further, quality defects encountered in the existing rolling process and occurring at the leading and trailing portions of the steel sheets may be significantly reduced.
In the process of hot rolling a steel sheet, it is necessary to increase the service life of a working roller by reducing the friction between the working roller and the steel sheet. The contact area between the working roller and the steel sheet is lubricated for this purpose. Water injection is used for such lubrication.
In the water injection method, an oil-water mixture in which a small amount of lubricant is diluted in a large amount of water is directly injected onto the working roller, or injected onto a reinforcing roller. When the mixture is injected onto the reinforcing roller, the mixture is transferred onto the working roller, which rotates together with the reinforcing roller through contact with the same, such that the mixture that comes to be coated on the working roller is ultimately supplied to the steel sheet that is being rolled. The lubricant included in the mixture is supplied to a circular arc gap between the steel sheet undergoing pressing and the working roller in contact therewith.
In order to spray a suitable amount of lubricant on a surface of the working roller, air atomizing and steam spraying may be used in addition to water injection. Furthermore, the lubricant used for rolling purposes may be a liquid lubricant, a gel grease, or a solid lubricant.
The lubrication method in hot rolling is performed as in the following and consideration of the limitations imposed as a result of performing rolling in batches. 1) Since slipping occurs when an excessive amount of lubricant is used, it is necessary to perform control such that the supply of lubricant is optimized. 2) Even when a single steel sheet is lubricated, the lubricant is not supplied to the entire length of the steel sheet being rolled, but rather, is supplied to only an area of the steel sheet excluding select sections of leading and trailing portions of the coil. 3) Lubricant is not continuously supplied, and instead, an intermittent lubrication method must be employed in which switching between supply and cut off is performed. 4) If an excessive amount of lubricant is supplied, a friction coefficient between the steel sheet and the working roll is disproportionately reduced such the steel sheet may not be able to pass through the rolling equipment. For these reasons, the lubrication method in conventional hot rolling is such that lubrication may only be passively performed.
Endless hot rolling, in which hot-rolled steel sheets are continuously connected at a front of a finishing mill and rolled, improves productivity as a result of reducing idle time. However, in endless rolling, the load on the working roller is increased when the reduction ratio and rolling speed are increased. As a result, heat fatigue of or wear on the roller occurs such that it becomes difficult to perform continuous rolling.
Japanese Laid-Open Patent No. 3-128113 is an example of a technique for supplying lubricant to a working roller in hot rolling. A lubricant supply apparatus is disclosed in this publication.
In this lubricant supply apparatus, a lubricant is mixed in warm water, and a nozzle is used to supply the mixture to a working roller. Further, in this lubricant supply apparatus, the lubricant and warm water are separately supplied through lines, and after the lubricant and warm water are mixed in a mixer, the mixture is sprayed onto the working roller through a spray nozzle mounted on a nozzle head. In addition, the supply of the lubricant may be cut off and only warm water supplied through the nozzle for spraying onto the working roller. Also, with the use of this lubricant supply apparatus, nozzle heads may be horizontally mounted spaced apart at equal intervals along the direction of the width of the steel sheet such that lubricant may be supplied to the entire width of the steel sheet.
In this lubricant supply apparatus, the nozzles mounted according to the width of the steel sheet may be individually controlled such that lubricant can be supplied to correspond to various different types of steel sheets of different widths. Lubricant and cold water are mixed immediately prior to spraying. Accordingly, after mixing and until spraying occurs, since the mixture passes through a short pipe, oil-water separation does not occur, and spray timing is excellent. However, there is a significant distance between a storage tank and the nozzle(s), thereby necessitating the use of a precise pump to control small amounts of lubricant. Hence, a precise oil pump is needed for each of the nozzles. Since the pipes are long and a large number of the nozzles are required in the lubricant supply apparatus, the piping is complicated. In addition, since pipes having a small diameter must be used, pressure loss of the pipes occurs, thereby making installation and maintenance of the equipment difficult.
In the conventional lubricant supply method, a precision metering pump is used to draw the lubricant from a storage tank to a mill stand. A mixture resulting from mixing the lubricant with cold or warm water is then sprayed onto the working roller through a nozzle head. In addition, in this method, spray timing is adjusted using a control valve mounted on the pipe that controls the supply and cut off of the mixture to the nozzle. However, while the equipment is simple in this method, as a result of the high temperatures involved in hot rolling, the mixer for mixing the lubricant and water and the control valve must be separated from the nozzle head by a considerable distance.
As a result of this structural problem, oil-water separation and spray timing delay problems are encountered with this method. Further, since a large amount of water is supplied when the lubricant and water are mixed, differences in the concentration of the mixture occur. In some instances, this may result in the lubricant not adhering well to the working roller.
Japanese Laid-Open Patent No. 2002-282911 discloses a technique for solving the problem of lubricant not adhering well to the surface of a roller due to differences in a concentration of a mixture. In the method of this patent, although an area of a pipe of a mixer for mixing water and lubricant is adjusted, the manner in which the area of the pipe is adjusted is not specifically disclosed.
Lubricant sprayed in accordance with a width of a steel sheet must be uniformly supplied by an amount that is in proportion to the width of the steel sheet. However, in the conventional lubricant supply method, differences in the concentration of the lubricant mixed with water according to spray area occur. Further, if a spray area is separated into a narrow region and a wide region for the supply of lubricant, although the equipment is simplified, mixture is wasted so that the consumption of lubricant is increased. In addition, if the spray area is subdivided, although it is possible to perform more precise control, the number of control valves is increased and the equipment becomes complicated.
In another lubricant supply method, lubricant amount is controlled individually by a nozzle and a supply pump. However, since small amounts of lubricant must be precisely controlled in this method, it is extremely difficult to dilute the lubricant at uniform concentrations in large amounts of water, and piping equipment as well as repair and maintenance are made difficult.
In yet another lubricant supply method, lubricant required for the entire roller is supplied all at once to a pump and diluted with water, after which the mixture is divided among nozzle heads. However, while the equipment is simple using this method, when lubricant is sprayed in a direction of a width of varying steel sheets, differences in a concentration of the lubricant along the width of the steel sheets are encountered.
In still yet another lubricant supply method, premixing is used. In this method, after water and lubricant are mixed in a mixer, the resulting mixture is divided among and supplied to nozzles mounted in each of the spray areas. However, although this method is suitable for use in rolling in which steel sheets are rolled one at a time, if it is necessary to perform lubrication when rolling steel sheets of differing widths, variations occur in the concentration of the lubricant along a width of the steel sheets to thereby result in uneven lubrication.
Japanese Laid-Open Patent No. 2002-282911 discloses a method for solving these problems. In this publication, areas of lubricant supply pipes connected to nozzles of each area are varied. However, the manner in which the areas of the pipes are adjusted is not specifically disclosed.
Accordingly, the conventional lubricant supply methods and apparatuses have many drawbacks that must be overcome to allow for suitable application to endless hot rolling in which steel sheets of different sizes and materials are connected and rolled.
The present invention provides an apparatus and method for supplying lubricant in endless hot rolling equipment, in which flaws on a surface of a steel sheet due to sticking between a working roller and a steel sheet during endless hot rolling are prevented, and wear in the working roller is minimized.
The present invention also provides an apparatus and method for supplying lubricant in endless hot rolling, in which continuous rolling may be performed by spraying lubricant in accordance with varying widths of steel sheets.